Fix issue with empty string values in Python 3.3

In Python 3.3 if string (unicode) table elements are left uninitialized
then subsequent attempts to get the string repr can produce a seg
fault.  This can occur even if the value is masked out.   See:
https://github.com/numpy/numpy/issues/3276

This patches initializes the values to zero instead of empty for
masked arrays.  For regular numpy arrays all values get filled
with something meaningful.

Note that the join() function works a bit differently and doesn't
suffer from this problem.